I have a nice cordless drill that came with two rechargeable batteries. However, there are times when life gets in the way and I'm not using the drill at all over the course of several months. When I return to the drill, both of the batteries have died.

How should I store the batteries when they are not going to be used for several months? Is there a way to do so such that they'll have a charge when I need them next? Are there any steps I should be taking when storing the batteries so lengthen their overall lifetime?

4 Answers
4

Batteries in general have several points worth concerning their usage. Yes, you are asking about storage but it is worth knowing that their storage strategy is influence by their usage frequency as well. Which type you have is important. There is not one overall method.

Li-Ion (Lithium Ion)

Like most batteries they should be stored at room temperate and not in direct sunlight. Some sources mention that storing it slightly warmer can help it perform better but the consensus is storing it at 15°C (59°F) is ideal. A dry area is also an important factor.

In general we are advised to store the battery in a partial discharge state. Storing the battery in either a fully charged or discharged state can actually harm the battery. Most manufacturers recommend storing these batteries in a 30-40% charge level. Charge level can be determined usually by battery temperature. The better you take care of it the longer it will last. I don't see myself ever testing the charge level but since I use my drill a lot I get a feel for when its getting down in charge since it starts to lose its torque.

The above tactic is sound but it is important to fully discharge the battery every 30 or so cycles of use to reset the batteries Digital Memory

The digital memory effect is a failure mode whose effect results in the transmission of improper calibrations of the battery’s fuel gauge to a device.

To that effect the article goes on to say that

To correct the digital memory effect and properly re-calibrate the fuel gauge circuitry simply do a full cycle discharge/recharge every several dozen charges. There is no real hard number.

NiCd/NiCad (Nickel-Cadmium)

Similar temperature and moisture suggestions exist for NiCd in that they should be stored in a cool and dry location. Between −20 °C and 45 °C (-4°F - 113°F) is recommended but I have seen several sites suggesting that freezing should be avoided ( Guessing mostly due to potential ice crystal build up. This could be avoided by putting the battery in an air tight bag.)

The batteries themselves should be stored either fully charged or fully discharged. Note that they have a higher discharge rate when compared to Li-Ion batteries but are not permanently affected by this discharge.

If you plan on storing these batteries long term (more than a couple of months) it is important to use them periodically to prevent crystals from forming and shorting the cells. The crystals can lower battery performance and the extreme one can cause damage that is irreversible. While modern NiCd's don't have a true internal memory problem, the crystal formation factor can effect the battery in a similar way.

NiMH (Nickel Metal Hydride)

Depending on what you read these batteries had a rough start when used for power tools. They have been getting better but were not as widely accepted until more recently due to their disadvantages.

As far as temperature and periodic use the NiMH and NiCd have the same approach. The important difference is that NiMH have the highest discharge rate and it is more important to periodically use it to prevent damage. Some people have made special cradles to allow a trickle charge to prevent this effect although that is more for your standard AA,AAA batteries.

Refer to your manual

Assuming you still have it there could be more specific instructions included with your device.

Awesome resource

I have been reading a lot about this and this website has information that covers a broad overview and more in-depth coverage of batteries if you so chose.

Good answer (upvoted), but one minor point: battery temperature is a very inaccurate way of determining charge level. A multimeter will tell you voltage. Halfway between "dead" and "fully charged" is probably optimum for Li-ion.
– Aloysius DefenestrateJun 13 '15 at 4:07

It sounds like your drill has Nickel Cadmium (also called NiCd or NiCad) batteries. This type of battery self-discharges very rapidly and will not hold a charge as long as a Nickel-Metal Hydride (NiMH) or Lithium-based battery (such as Lithium-ion or Lithium-polymer).

Matt's answer includes a lot of technical detail on how to best store and maintain your batteries. I won't repeat that information but I will mention a few very noteworthy points regarding each of the three battery technologies commonly used for cordless tools.

When not under load or charge, a Ni–Cd battery will self-discharge
approximately 10% per month at 20°C, ranging up to 20% per month at
higher temperatures. It is possible to perform a trickle charge at
current levels just high enough to offset this discharge rate; to keep
a battery fully charged. However, if the battery is going to be stored
unused for a long period of time, it should be discharged down to at
most 40% of capacity (some manufacturers recommend fully discharging
and even short-circuiting once fully discharged), and stored in a
cool, dry environment.

You'll get more recharge cycles out of Lithium-ion (Li-ion) batteries if you recharge them once they drop to 20% capacity, and as Matt mentioned, you should store them at 30-40% charge.

NiMH batteries for tools are not very common in the US. Older NiMH technology had an internal discharge rate of about 3% per month, which is better than NiCd but not as good as Li-ion. Newer NiMH batteries have a very low internal discharge rate and can retain a charge better during long-term storage.

What does this mean in practical, real-world terms?

All this talk about storing batteries at 40% charge or topping off Li-ion batteries at 20% charge is almost purely academic for various reasons. You can have the best intentions of following the guidelines, but it's a pain in the neck to try to adapt your own behavior to suit the battery technology. The best advice I can give is to just buy a battery technology that doesn't require a babysitter. In the US, that's probably Li-ion. Outside the US, it may be Li-ion or NiMH.

As you said in your question, "there are times when life gets in the way...." Unless you're able to dedicate significant time to woodworking on an ongoing basis, you may not know when you'll use a tool again so it's impossible to know whether you should prep your batteries for long-term storage.

Some batteries or tools do have integrated meters which show you the battery's current charge level. One of my Li-ion drills has a 3-bar indicator but it isn't particularly accurate--by the time the tool reaches 1 bar, the battery is practically dead. If your tools or batteries don't have accurate charge meters or don't have any meters at all, good luck figuring out if you're charging your batteries at the right time or storing your batteries at the optimal storage capacity.

I've owned drills and other types of devices with both types of batteries that are common in the US, NiCd and Li-ion.

Every time I've gone to use my NiCd drill, it has been dead or it didn't last long enough to finish the job. I basically had to charge it every time I wanted to use it, and that takes several hours. Usually I would end up with an unplanned window of time during which I want to get something done, and that window of availability was used up just waiting for the battery to charge. When I did get to use the drill, I sometimes would deplete one battery and put it on the charger while I switched to the other battery and continued working. By the time I drained the second battery, the first one was still barely charged. Granted, there are more advanced NiCd batteries and chargers that allow for faster charging, but they are not always available for a given line of cordless tools. If they are available, they often aren't included and have to be purchased separately--in which case, you perhaps could have just chosen a more hassle-free type of battery.

In contrast, when I go to use my Li-ion drill, the battery loaded in it is always ready to go and has at least some charge, and the second battery is ready to go as soon as the first one runs out of juice. The batteries only take about 30 minutes to charge and last a long time. As soon as the first one is drained, I can put it on the charger and I know it'll be fully-charged by the time the second one runs out of juice. I've had the drill (and batteries) for several years now and haven't noticed any degradation of the batteries.

Summary

In just a few words, NiCd batteries require a lot of babysitting, NiMH batteries are better, and Li-ion are the lowest-maintenance.

No batteries do very well in extreme heat or extreme cold. During the summer, I store my batteries in my garage shop and try not to leave them in my car if I need to take them elsewhere. If your shop gets extremely hot, you may want to store your batteries someplace cooler. During the winter, I store my batteries inside my house and don't leave them in the garage or in my car to prevent them from freezing.

If you use your tools every day or even every week, NiCd might work for you. If you regularly go more than a month without using your tools, it's probably safe to say you sometimes go several months without using your tools, and you'd often need to charge your NiCd batteries before you can use them. In that case, you're probably best served by Li-ion or NiMH batteries.

Fortunately, many lines of tools now offer compatible batteries in both NiCd and Li-ion types. If you already have a tool with NiCd batteries but NiCd is not suitable for your work patterns, you may be able to "upgrade" to a Li-ion battery by buying a compatible replacement Li-ion battery (and Li-ion charger), or by buying a new tool whose batteries are compatible with your existing tool.

Do you think it might be better to compare the batteries in another question. This one is getting a little long. I was trying to avoid comparing them. Extra information is always good but a question about "What are the different types of batteries for cordless tools?" might serve better. What do you think?
– MattApr 13 '15 at 20:20

Ha, I didn't add that much between your "Great answer" comment and the "hold on, this is getting too long" one! But you may have a point; let me think about it. My intention was to answer the question from a practical angle but incidentally I added the comparisons for clarification.
– rob♦Apr 14 '15 at 4:55

Your addition did not influence the second comment. I was thinking about my quarter-sawn answer and how one question was asked but i started to answer another. It made me reread this and think about the scope of the OP.
– MattApr 14 '15 at 14:24

Welcome to SE. If your read the original Question fully the OP specifically mentions that he might not come back to the tool for several months. It's neither practical nor safe to leave many batteries in their chargers for extended periods, not even a few days..
– GraphusAug 1 '18 at 11:37

Can you explain why the garage would be worst? Is your garage heated?
– Maxime MorinApr 13 '15 at 17:10

It has to do with the temperature fluctuation. I do have a heater in the garage but run it only when I'm in there. It's attached so it's warmer than outside but still drops pretty low. There is no method for cooling.
– Dano0430Apr 13 '15 at 18:56